If the value in the date_time column is not null, then the value in the time_taken column is null, which gets turned into undef I guess.

No, time_taken is always fine, date_time is not coming back from the get_column request, even though it's there.

The report is selected and ordered by date_time, so if the date_time value was actually null it wouldn't be in the result set, or they'd all be at the end or the beginning, rather than what I'm seeing, which is the occasional row failing to populate date_time in the results.

Now, looking at your actual output, I see that indeed, date_time cannot be NULL because you order by it. So it must be a bug or a problem with either DBIx::Class or your database driver. I don't know if it's easy to get at the unsullied result set as returned by DBI, but maybe you can determine the culprit by doing:

my $dbh = DBI->connect(..., {RaiseError => 1});
my $sth = $dbh->prepare(<<SQL);

    SELECT me.id, me.name, me.date_time, me.status_code, me.time_taken
+ 
    FROM tests me
    WHERE ( ( ( date_time <= ? AND date_time >= ? ) AND name = ? ) )
    ORDER BY date_time

SQL

$sth->execute('2009-06-24 23:59:59', '2009-06-24 00:00:00', 'ART_CRV')
+;
my $res = $sth->fetchall_arrayref();

print Dumper($res);
[download]

If you find any undef in there, the problem is with your DBD. If you don't find any undef there, the problem is likely in your DBIx::Class stack somewhere.